In a satellite telephone interview with the BBC from an undisclosed location, Mr Machar said a mechanism was required to monitor any ceasefire.

"For the ceasefire to be credible there is need for a mechanism, or else we will be deceiving ourselves," he said.

He also demanded that Mr Kiir release all 11 of his political allies who were arrested right at the beginning of the unrest, while acknowledging that two of them had been freed.

The regional leaders brokering the end to hostilities have given the two rivals four days to hold face-to-face talks and halt fighting, pledging unspecified "further action" if the civil war continued.

Meanwhile the UN said dangers remained so acute that large numbers of bodies had been seen outside at least one UN base, but could not be collected.

A 72-member UN police contingent arrived in the country on Friday, according to a UN spokesman in New York. They are the spearhead of what is to be 6,000 extra troops authorised by the UN Security Council to bolster the hard pressed UN mission.

More troops and equipment were expected to arrive on Saturday.

Once all the reinforcements are in, they will almost double the size of the UNMISS mission in the country to a total of up to 12,500 soldiers and 1,300 police.

That mission has so far been badly stretched as fighting has claimed thousands of lives since erupting December 15.

The UN says more than 120,000 people have fled their homes, including 63,000 sheltering in UN peacekeeping bases, and neighbouring nations and world powers feared South Sudan was sliding into civil war.

Since it started, the fighting has spread to half of South Sudan's 10 states, with the violence taking on an ethnic dimension, pitting members of Mr Kiir's Dinka tribe against Mr Machar's Nuer community and atrocities reported to have been carried out by both sides.

The warring forces are locked in fierce battles for control of several strategic oil-producing areas in the north of the country, which only won independence from Sudan in 2011.

Witnesses have reported heavy clashes in Malakal, capital of the oil-producing Upper Nile State, which both sides claimed to control.

A rebel spokesman in the area, Moses Ruai Lat, told AFP that "the whole of Malakal" was now in the hands of Machar's loyalists.

But South Sudanese Defence Minister Kuol Manyang Juuk dismissed that claim as "disinformation" and asserted that Machar's loyalists "are no longer in Malakal - the town is under full government control".

Rival sides have also massed their forces around Bentiu, a key city in the oil-producing state of Unity as well as Bor, the capital of Jonglei state.

"Government forces are believed to have consolidated their positions in and around Bor," said a UN statement. "Anti-government forces remain in the vicinity and the situation remains tense."

UN aid workers only returned to Bor on Thursday to help displaced civilians, said the UN's Office for the Coordination for Humanitarian Affairs (OCHA).

"Following the intensive fighting for the town, there are reportedly a large number of bodies in the open near the base, which poses a risk for disease outbreaks," the office added.

The United Nations has rushed "critical assets" to the country, where it says it is struggling to cope with the dual role of protecting as well as feeding and sheltering terrified civilians.

Crude prices have edged higher because of the fighting as oil production, which accounts for more than 95 percent of South Sudan's economy, was dented by the violence and oil workers evacuated.